1. Field of the Invention
The present invention relates to a method for nondestructively evaluating and quantitatively finding deterioration of material strength due to aging of an austenitic stainless steel structural material or an austenitic stainless steel structure comprised of such structural materials.
2. Description of the Related Art
Since more than half a century ago, it has been known that austenitic stainless steel transforms from paramagnetic to ferromagnetic, due to plastic deformation. Further, nondestructive evaluating methods utilizing this phenomenon have been attempted in U.S.A., German, Japan and other countries. Thus, as methods for nondestructively evaluating “deterioration of material strength due to aged metal fatigue of an austenitic stainless steel structural material or an austenitic stainless steel structure comprised of such structural materials” (hereinafter called “aged deterioration of strength”), there have been resultingly and conventionally-known those methods which adopt a measuring device for measuring saturation magnetization of martensitic phase transformed from an austenitic stainless steel due to plastic deformation thereof, so as to evaluate aged deterioration in the austenitic stainless steel. Moreover, there has been known a method for evaluating fatigue of an evaluation-target material (i.e., aged deterioration of strength) based on a change of magnetic permeability of the material, because the magnetic permeability of the material changes with transformation from austenite phase to martensitic phase due to plastic deformation (see JP-A-8-248004, for example).
However, it has been impossible to uniquely obtain information of an internal factor (internal stress), by the conventional method for evaluating an aged deterioration in austenitic stainless steel based on magnetic characteristics such as saturation magnetization and magnetic permeability.
Namely, in the above-mentioned conventional nondestructive evaluating method for evaluating aged deterioration in an evaluation target, there are obtained reference data by performing magnetization measurement by changing the amount of external factors (external stress, temperature) which cause the martensitic transformation (for example, by changing the number of times of repeated load in the above-mentioned JP-A-8-248004) so as to change the amount of the martensitic phase deriving from austenite phase while utilizing a fact that the martensitic transformation is caused by a machining operation of the evaluation target, to thereby nondestructively evaluate the aged deterioration in the evaluation target based on the reference data.
The martensitic transformation is caused by various factors, and the martensitic transformation is caused by not only external factors (external stress, temperature) but also internal factors (internal stress, chemical composition).
However, those magnetic characteristics, such as saturation magnetization and magnetic permeability which have been conventionally used as parameters for evaluating aged deterioration, are changed in response to the amount of the martensitic phase which is changed by the amounts of external factors, and thus the magnetic characteristics are not necessarily brought into a one-to-one relationship with the internal factor (internal stress) which is a cause of the aged deterioration of strength. Thus, it has been impossible to precisely obtain information of lattice defects such as dislocations within a material by the above-mentioned conventional nondestructive evaluating method for aged deterioration in austenitic stainless steel based on magnetic characteristics such as saturation magnetization and magnetic permeability.